1. Field of the Invention
The present invention relates to a method of controlling a static magnetic field in an NMR spectrometer in such a way that the temperature inside a sample tube can be controlled.
2. Description of Related Art
From the past, field gradient shimming has been known as a technique for improving magnetic field homogeneity in an NMR sample space by the use of magnetic field gradients (see, for example, J. Magn. Reson. 125, 197-201 (1997), A Practical Method for Automated Shimming with Normal Spectrometer Hardware, H. Barjat, P. B. Chilvers, B. K. Fetler, T. J. Horne, and G. A. Morris). In this method, magnetic field gradients are applied, and the resulting NMR signal is measured. Shim values for homogenizing the static magnetic field are calculated from the obtained intensity distribution of the static magnetic field.
A known technique consisting of blowing heated gas upward from the bottom side of a sample tube is available as a method of controlling the temperature of a sample in an NMR spectrometer. If the sample temperature is controlled by this technique, the temperature at the bottom of the sample tube is close to the target temperature. The temperature becomes closer to the room temperature approaching the sample top. Therefore, a temperature gradient is created along the height of the sample tube. If gradient shimming is performed under this condition, it is impossible to achieve a uniform static magnetic field because the chemical shift varies from position to position within the sample tube due to the temperature gradient. Especially, when gradient shimming is performed on a solvent having a chemical shift with large temperature dependence, a nonuniform static magnetic field is produced by the influence of the temperature gradient. Examples of solvent having a chemical shift with large temperature dependence include water, methanol, ethanol, isopropanol, DMF (N,N-dimethyl formaldehyde), and their deuterated solvents.
One method of solving this problem is to reduce the temperature gradient across the sample tube (see, for example, JP-A-2002-168932). However, it is difficult to make uniform the temperature across the sample tube while avoiding deterioration of other performance characteristics. Another method is to replace the solvent by a solvent having a chemical shift with less temperature dependence. If different solvents are used, it is difficult to directly compare different sets of data. Furthermore, where the observed subject is insoluble, this method cannot be used. A further method is to perform a measurement at a controlled high temperature after performing shimming at room temperature. However, variations in the magnetic field caused by temperature variations cannot be canceled out.